Temperature Compensated Crystal Oscillator (TCXO) enables stable output frequencies in response to changed in ambient air temperatures. Therefore, TCXO is widely used for a mobile telephone, a Personal Navigation Device (PND), and the like.
In recent years, higher functions and longer life of operable period are demanded for these devices, whereas their mounted components are required to lower the power consumption. Conventionally, the low power consumption has been promoted by sampling action of TCXO. In order to achieve further lower power consumption, however, operation of TCXO at low voltages is requested. As a conventional oscillator with such a temperature compensated crystal oscillator, there is an example of an oscillator in Patent Document 1.
FIG. 7 is a configuration diagram illustrative of main components of an oscillator described in Patent Document 1. This oscillator is formed of: an oscillation circuit 1 capable of changing the frequency depending on the voltage; and a bias generation circuit 2 for generating bias signals necessary for driving the oscillation circuit 1.
Herein, the bias generation circuit 2 corresponds to an approximate cubic function generator or the like described in Patent Document 1.
FIG. 8 is a configuration diagram illustrative of an approximate cubic function generator described in Patent Document 1. For example, in a case where an AT-cut crystal oscillator is used, its oscillation frequency has temperature characteristics approximated by a cubic function. Hence, such temperature characteristics can be cancelled by an approximate cubic function generator 21.
The approximate cubic function generator 21, upon receipt of a temperature detection value as an input signal VIN changing in a primary function manner with respect to the temperature change from a temperature detection circuit 22, generates a temperature compensation voltage (bias signal BIAS) for compensating the temperature characteristics of crystal to supply BIAS to the oscillation circuit 1. The approximate cubic function generator 21 is formed of: an adder 23; a cubic component and constant component generator 24; a primary component generator 25; and an adder circuit 26. The adder 23 adds a variable voltage V0 for adjusting the center temperature of the cubic curve to the input signal VIN, and then outputs an added output VS. The added output VS is input into the cubic component and constant component generator 24, and an output signal VAOUT is output therefrom. The added output VS is input into the primary component generator 25, and an output signal VBOUT is output therefrom. The adder circuit 26 adds the output signal VAOUT and the output signal VBOUT, and then outputs the temperature compensation voltage (bias signal BIAS). The cubic component and constant component generator 24, the primary component generator 25, and the adder circuit 26 constituting the approximate cubic function generator 21 receive supply of the power from an external power supply for supplying the power supply voltage VDD.
The bias generation circuit 2 generates the temperature compensation voltage (bias signal BIAS) for driving the oscillation circuit 1, and the reference voltage or reference current necessary for driving the oscillation circuit 1 to be supplied to the oscillation circuit 1. This allows precise compensation of the temperature characteristics of the crystal oscillator.